1. Field of the Invention
This invention relates generally to humidity indicators for containers, and more particularly to a thin flexible color change humidity indicator of sandwich construction installed in the side wall of a flexible container.
2. Brief Description of the Prior Art
Thin flexible water vapor barrier containers, such as envelopes, bags, and packages are commonly used in the electronics industry to store and ship moisture sensitive electronic components. Such containers, with the moisture sensitive components enclosed therein, are usually stacked in storage bins or boxes. They are also shipped to various destinations through the mail or via commercial carriers and the container and its delicate contents are subjected to rough handling often in high humidity environments.
Often, moisture sensitive elements, such as electronic components, are damaged due to exposure to humidity through damaged packaging or improper packaging. Many electronic components can also be contaminated by particulate debris which can enter through a damaged package or which can even be caused by particulate matter of the package itself.
Therefore, it would be desirable to provide a thin flexible, humidity indicator suitable for use in the wall of a flexible water vapor barrier container which will visually indicate varying levels of humidity within the container and has a particulate filter between the humidity responsive element in the indicator and the contents of the container and which will allow stacking of flexible containers for shipping and storage without damage or contamination of the contents.
Dehydrated packaging was developed during the second World War to protect metallic, organic, and synthetic materials from deterioration due to high relative humidity in the Pacific and the tropics, and was improved by placing a predetermined amount of an indicating desiccant silica gel inside the package which would indicate an unsafe condition. However, once the silica gel showed an unsafe condition, it could only be used again if it was reconditioned (by heat or otherwise).
Color change chemical indicators were developed after the war which used blotting paper coated with various solutions of cobalt chloride and additives. The blotting paper/cobalt chloride indicators were placed inside the container and some containers were provided with a window so that the indicator could be seen without opening the container.
There are currently two basic forms of humidity indicators in the art which are suggested to be mounted in the walls of product packaging: a relatively expensive reusable type, and a relatively inexpensive disposable type. Conventional reusable types of humidity indicators consist of a rigid metal or plastic plug having a visual indicator and window in the head of the plug which is installed in the wall of the container. These plug type indicators may be used in both flexible and rigid containers.
Humidial Corporation of Colton, Calif. manufactures a disposable blotting paper/cobalt chloride indicator card which is placed inside of a container, and also a reusable rigid metal or plastic plug which has a window and a blotting paper indicator card installed in the face of the plug. The window type rigid plug indicator is a hollow bolt-like configuration having a threaded shaft and may be installed on flexible or rigid containers using hand tools and a gasket and locknut.
West, U.S. Pat. No. 3,425,388 discloses a disposable rigid window plug type humidity indicator which can be installed in either rigid or flexible plastic containers. West teaches a rigid plastic cup-like inner member having a flange on the bottom end and a peripheral bead at the top end, a transparent plastic cup-like outer member which snap fits onto the inner member, and a humidity responsive color change element which is clamped therebetween. The inner member flange is heat sealed to the package interior and its upper end extends through an opening in the package. The transparent outer member serves as a sight window and the inner member has an opening to the inside of the container to provide free communication between the container interior and the humidity responsive element.
Each of the above described humidity indicators present problems when used on flexible water vapor barrier containers and packages whether they be plastic or fiber and particularly in their use with flexible water vapor barrier bags of the type used in the electronics industry to store and ship moisture sensitive electronic components.
Due to the rigidity and thickness inherent in the design of the above described rigid humidity indicators when they are installed in a flexible container they will deform the container outward and add thickness and bulk to the area of the container at which they are installed. The rigidity of the inner fastener members can damage components inside the container. Should the flexible containers be stacked for storage or shipment, the cumulative weight and thickness of the indicators could seriously deform and/or displace the contents of the flexible container.
The opening in the inner member of the above mentioned indicators is intended to provide free communication between the contents of the container and the humidity responsive element and there is no provision of a particulate barrier between the humidity responsive element and the contents. Thus, there exists the possibility of particulate contamination of the contents from the humidity responsive element, which is composed of blotting paper. Blotting paper sheds paper fibers and lint. Also, the blotting paper contains a chemical solution of cobalt chloride and additives which will cause chemical contamination and/or corrosion of the contents should they come in contact with the indicating element itself.
Chemically treated blotting paper cards have been placed inside sealed opaque packages or containers, but this method also presents the problem of contamination of the contents by chemicals and debris from the card and requires that the package or container be cut open for inspection and then resealed.
The present invention is distinguished over the prior art in general, and the West patent in particular, by a thin flexible humidity sensor and indicator of sandwich construction sealably mounted in an opening in the wall of a container which provides a visual indication of the humidity within the container. The present humidity sensor and indicator has an intermediate layer of flexible humidity sensitive material chemically treated to change color corresponding to exposure to predetermined levels of humidity, a thin flexible layer of transparent water vapor barrier material on one side, and a thin flexible layer of water vapor permeable material on its opposite side which are sealed together and sealably secured over the opening in the container wall. The water vapor permeable layer is disposed on the interior of the container and provides communication between the container interior and the humidity sensitive layer and the transparent water vapor barrier layer and the humidity sensitive layer are visible from the exterior of the container to provide a visual indication of the value of the humidity within the container. In a preferred embodiment, the humidity sensitive vehicle is a sheet of blotter paper treated with a chemical solution of cobalt chloride and additives and the water vapor permeable material is flash spun, film fibril high density polyethylene material which prevents lint and debris from the blotter paper from entering the interior of the container and prevents direct contact between the chemically treated blotter paper and the contents of the container.